THE EVOLUTION OF THE EYE. 85 



only begin countless generations after that in the eyes of the 

 female. 



In the case of deep sea fishes Dr, Giinther says : " The organ 

 of sight is the first to be affected by a sojourn in deep water. 

 Even in fishes which habitually live at a depth of only 80 fathoms, 

 we find the eye of a proportionately larger size than in their 

 representives at the surface. In such fishes the eyes increase in 

 size with the depth iiihabited by them down to the depth 0/200 

 fathoms." 



Dr. Giinther had previously said that the rays of the sun 

 probably do not penetrate to, and certainly do not extend beyond 

 the depths of 200 fathoms. 



He continues : " Beyond that depth small as well as large-eyed 

 fishes occur, the former having their want of vision compensated 

 by tentacular organs of touch ; the latter, the large-eyed fish, can 

 only see by the aid of phosphorescence. In the greatest depths blind 

 fishes occur, with rudimentary eyes, and without special organs of 

 touch. Many fishes of the deep sea are provided with more or 

 less numerous round, shining, mother-of-pearl coloured bodies 

 imbedded in the skin. These, when large, are placed on the 

 head, in the vicinity of the eye ; the smaller ones are arranged in 

 series along the side of the body and tail. The former kind of 

 organs possess, in the interior, a body like the lens of an eye, and 

 are considered by some naturalists to be true organs of vision, 

 developed to catch the phosphorescent rays emitted by numerous 

 deep sea organisms." The functions of the globular bodies 

 arranged along the sides of the fish are at present unknown, but 

 Dr. Giinther thinks there is no doubt that the functions of these 

 organs have relation to the peculiar conditions of light — wholly 

 phosphorescent — under which these fishes live. 



I now come to a case of extraordinary development of the visual 

 organs. Upon many of the coasts of the Pacific Ocean, is found 

 a mollusc of the genus Onchidinm. This mollusc has eyes of the 

 ordinary invertebrate type placed upon its head. But the greater 

 number of species of this genus have other eyes situated on the 

 shell-less but rough back of the animal. These eyes, simple as 

 they are in structure, are extremely interesting, for they are 

 identical in type with those of the vertebrate. It is the only 



